1. Field of the Invention
This invention relates generally to a method for fabricating a sensor, and more particularly to a bulk silicon micromachining process for fabricating a sensor having a mass suspended from a support frame by high precision suspension springs.
2. Description of the Prior Art
Silicon is a material having highly desirable mechanical properties. Single-crystal or monocrystalline silicon is harder than most metal and is surprisingly resistant to mechanical stress. In both tension and compression, silicon has a higher elastic limit than steel and remains strong under repeated cycles of tension and compression below its elastic limit. So-called micromachining processes allow silicon to be made into mechanical devices of extremely small size allowing miniaturization of such devices in much the same way as has been the miniaturization of electronic devices such as computer chips, etc. Micromachining processes allow many devices to be fabricated at the same time so that the cost per device is low where many devices are fabricated. Micromachined devices are uniform in quality, because the same processing steps operating on a device on one side of a silicon wafer are the same as those on other parts of the wafer.
Micromachining normally utilizes chemical etching techniques for forming three dimensional shapes, such as pits, holes, trenches or walls. Micromachining usually begins with photolithography, a photographic technique used to transfer copies of a master pattern onto the surface of a silicon wafer. The first step is to grow a thin layer of oxide on the wafer surface by heating it to between 800 C. and 1200 C. in an atmosphere of steam. Then, a photoresist, a thin layer of an organic polymer sensitive to ultraviolet radiation, is deposited on the oxide surface, and exposed and developed. A pattern of bare and photoresist coated oxide is formed on the wafer surface. After subsequent treatment with acid, the oxide is etched away while the photoresist coated oxide remains. The final result is a pattern of oxide on the wafer surface that duplicates the photoresist pattern.
The oxide pattern may serve as a mask during the doping of the wafer with impurities, such as boron or phosphorous. The oxide pattern is also used as a mask during chemical or anisotropic etching with the etching material attacking the silicon. Anisotropic etchants, which are also known as orientation-dependent, etch at different rates in different directions in the silicon crystal lattice and can form well defined shapes with sharp corners and edges.
Acceleration sensors, motion sensors and pressure sensors are examples of devices fabricated by micromachining. There are many others. Examples of micromachined acceleration sensors include the piezosensitive and capacitive types.
Sensors may include a central sensing mass supported within an outer support frame for relative movement by one or more suspension members extending between the mass and the outer support frame. Suspension members heretofore have comprised suspension springs, beams, hinges, or diaphragms to support the sensing mass with respect to the outer support frame. Such suspension members have been micromachined heretofore using a wet anisotropic etch, for example.
U.S. Pat. No. 4,922,756 dated May 8, 1990 shows a micromachined accelerometer fabricated from silicon by batch photolithographic techniques from a monocrystalline wafer. The accelerometer includes E-shaped leaf springs fabricated from silicon dioxide. The top silicon spring faces of the E-shaped springs on each side of the wafer are masked, and silicon dioxide is grown on surfaces that are later patterned and etched away by an chemical etching. A thin photoresist or stop layer is formed. A thick oxide layer is then grown by diffusing a precise amount of oxygen into the silicon to form a precise thickness of silicon dioxide on the wafer. Next, the opposite surfaces of the wafer are masked exposing selected areas which are etched down to the silicon surfaces. A thin silicon dioxide layer is then grown, masked, and the exposed silicon dioxide etched down to the silicon surface exposed to the silicon etchant. A chemical etch of the silicon material is then applied which acts through openings of the silicon dioxide to etch silicon material away while undercutting the legs of the E-shaped springs. Stiff etched stop layers remain for the bases of the springs and the opposite faces of the suspended mass and support structures. Force and sense conducting areas are then deposited on opposite faces of the suspended mass and on complementary force and sense conducting areas of the opposing support structures. Thus, three dimensional E-shaped springs are provided with the length of each leg and its thickness accurately controlled to control the spring constant of the sensing member.
U.S. Pat. No. 4,144,516 dated Mar. 13, 1979 is directed to a leaf spring sensor or transducer fabricated by batch photolithographic etching techniques from a monocrystalline silicon material. Two separate wafers are formed with an array of spring structures therein. Each wafer includes electrical circuitry on a top surface with the bottom surface etched with an anisotropic etchant to define an E-shaped leaf spring structure and surrounding supporting structure. Then, the two wafers are joined together such as by adhesive, soldering, or brazing to form a dual spring sensor or transducer. Individual transducers are then separated from the joined wafers and subsequently mounted to a suitable support structure.
Both of the spring systems described above include E shaped springs. The prior art of semi-conductor micromachining includes spring and suspension members having straight lines which are characteristic of the chemical etching process along crystallographic planes of diamond cubic materials such as silicon. The creation of spring geometries by chemical etching of silicon material accordingly is limited as to the shapes of spring and suspension members which can be micromachined.